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Explain why reheating foods does not always make them safe to eat.

If food is held at improper temperatures for enough time, pathogenic bacteria have the opportunity to multiply to dangerous numbers. Proper reheating kills most pathogens of concern, except for bacterial spores. It is especially effective in reducing the numbers of Clostridium perfringens that may grow in meat, poultry, or gravy if these products were held at improper holding temperatures or improperly cooled. However, proper reheating will not necessarily destroy bacterial toxins. For example, toxins such as that produced by the bacterium, Staphylococcus aureus, are heat-stable and so are destroyed only if the food is boiled for at least ten hours.

Pathogenic bacteria are more likely to grow in reheated cooked foods than they are in raw foods. This is because spoilage bacteria, which inhibit the growth of pathogens by competition on raw foods, are killed during cooking. Subsequent recontamination by pathogens will allow them to grow without competition from spoilage bacteria if temperature abuse occurs.